1. Field of the Invention
The present invention relates to a power supply apparatus.
2. Description of the Related Art
Various kinds of consumer electronics devices such as TVs, refrigerators, etc., or otherwise electronic devices such as laptop computers, cellular phone terminals, and PDAs (Personal Digital Assistants), are each configured to operate receiving electric power from an external circuit, and to be capable of charging a built-in battery using electric power received from an external power supply. Such consumer electronics devices and electronic devices (which will collectively be referred to as “electronic devices” hereafter) each include a built-in power supply apparatus configured to perform AC/DC conversion of commercial AC voltage. Alternatively, such a power supply apparatus is configured as a built-in component included within an external power supply adapter (AC adapter) for such an electronic device.
FIG. 1 is a block diagram which shows a configuration of an electronic device 1r including a power supply apparatus 2r according to a comparison technique investigated by the present inventors. The electronic device 1r includes the power supply apparatus 2r and various kinds of function circuits (loads) 4a and 4b such as a microcontroller 3 etc. Examples of the loads 4a and 4b include a display panel, a driver for such a display panel, an audio processing circuit, an image processing circuit, and so forth.
The power supply apparatus 2r includes a fuse 6, a filter 8 configured to remove noise from commercial AC voltage Vac, a rectifier circuit (diode bridge circuit) 10 configured to rectify the commercial AC voltage Vac, a power factor correction circuit (PFC circuit) 20r, a first insulation type DC/DC converter 30r, and a second insulation type DC/DC converter 40r. 
The PFC circuit 20r is configured to monitor the AC input voltage Vac and the input current Iac, and to control the phases of the input voltage Vac and the input current Iac such that they match each other, thereby maintaining the power factor in the vicinity of 100%.
The electronic device 1r such as a TV or the like has three states, i.e., a shutdown state in which all the functions are disabled, a standby state in which only minimal operations are enabled, and a normal operating state. For example, with an electronic device including two kinds of switches, i.e., a main power supply switch and a standby switch, the shutdown state corresponds to the state in which the main power switch is off, and the standby state corresponds to the state in which the main power supply switch is on and the standby switch is off. The normal operating state corresponds to the state in which both the main power supply switch and the standby switch are turned on.
The first insulation type DC/DC converter 30r is configured to supply the power supply voltage Vdd1 to the load 4a which is configured to operate only in the normal operating state. The first insulation type DC/DC converter 30r is configured to operate only in the normal operating state, and stops operating in the standby state.
However, there is a need to supply the power supply voltage to the microcontroller 3 configured to control the overall operation of the power supply apparatus 2r not only in the normal operating state but also in the standby state. In order to meet this need, the second insulation type DC/DC converter 40r is arranged in order to supply the power supply voltage Vdd2 to the microcontroller 3 (and other loads 4b as necessary), in addition to the first insulation type DC/DC converter 30r. The second insulation type DC/DC converter 40r is configured to always operate in the state in which the main power supply switch is on. A sub-converter using an auxiliary winding is provided to the primary side of a transformer included in the second insulation type DC/DC converter 40r. The output voltage Vcc of the sub-converter is supplied as the power supply voltage for the control circuits (controllers) of the PFC circuit 20r, the first insulation type DC/DC converter 30r, and the second insulation type DC/DC converter 40r. 
The power supply apparatus 2r shown in FIG. 1 requires the installation of such insulation type DC/DC converters as two separate circuits, leading to an increased number of components, resulting in an problem of increased costs. In particular, a transformer is costly.